New research from the May hurricane conference of the American Meteorological Society sheds new light just ahead of the start of the season June first.

Evidence linking solar variability with USA hurricanes, by Robert Hodges and Jim Elsner of Florida State University. They showed that the probability of three or more hurricanes hitting the U.S. during a hurricane season with warmer than average sea surface temperatures increases dramatically during minima in the 11-year sunspot cycle. The odds increase from 20% to 40% for years when the sunspot activity is in the lower 25% of the sunspot cycle, compared to years in the upper 25% of the cycle. Near the peak of the sunspot cycle, the odds of at least one hurricane hitting the U.S. are just 25%, but at solar minimum, the odds increase sharply to 64%. The authors studied the period 1851 - 2008, and controlled for other variables such as changes in sea surface temperature and El Niño. Such a large impact of the sun on hurricanes might seem surprising, given that the change in solar energy at all light wavelengths is only about 0.1%. This relatively small change causes just a 0.1°C change in Earth's mean surface temperature between the peak of the 11-year sunspot cycle (high solar activity) and the minimum of the sunspot cycle (where we are now.) However, variation in radiation between extrema of the solar cycle can be 10% or more in portions of the UV range (Elsner et al., 2008.) The strong change in UV light causes globally averaged temperature swings in the lower stratosphere of 0.4°C between the minimum and maximum of the sunspot cycle--four times as great as the difference measured at Earth's surface (Lean, 2009). This sensitivity of the stratosphere to UV light is due to the fact the ozone layer is located in the stratosphere. Ozone absorbs a large amount of UV light, causing the stratosphere to heat up when solar activity is high. The authors speculate that a warmer stratosphere then heats up the upper troposphere, making the atmosphere more stable. An unstable atmosphere--with hot temperatures at the surface and cold conditions in the upper troposphere--are conducive for stronger hurricanes. Thus, we would expect to see reductions in hurricanes during the peak of the sunspot cycle.